This invention relates to a substrate system for spray forming and an associated method.
Spray forming, also referred to as spray casting, of molten metal is a fairly well known process for producing various types of metal products. Spray forming consists of introducing a controlled stream of molten metal into a gas-atomizing nozzle where it is impacted by high-velocity jets of gas, usually argon or nitrogen. The resulting spray of metal droplets is deposited onto a substrate to form a highly dense product. See U.S. Pat. Nos. 4,938,278; 5,110,631; 5,143,140; and 5,154,219.
Various types of substrate problems exist in spray formed products. For example, one problem concerns a high degree of substrate layer porosity which is formed due to high chilling at the substrate layer at the leading edge of the spray. U.S. Pat. No. 4,917,170 recognizes the problem of substrate layer porosity and attempts to solve the problem by providing a low thermal conductivity substrate for use in the spray forming process. The substrate is made entirely of a low thermal conductivity material. U.S. Pat. No. 5,240,061 also recognizes the problem of substrate layer porosity in spray forming processes. A moving substrate in the form of a thin foil is provided to reduce and limit heat extraction from the particles being deposited on the substrate so as to reduce substrate layer porosity.
Another problem relates to inadequate heat extraction from a bulk layer of the sprayed metal droplets deposited on the substrate. Typically, slow cooling rates in the bulk layer results in coarse constituent particles and large grains. As can be appreciated, this problem is enhanced by substrate designs aimed primarily at resolving the problem of substrate layer porosity. In other words, a substrate system which reduces the problem of substrate layer porosity enhances the problem of inadequate heat extraction from the bulk layer, while a substrate system that reduces the problem of inadequate heat extraction from the bulk layer enhances the problem of substrate layer porosity.
What is needed, therefore, is a substrate system for spray forming molten metal droplets to form a metal product that effectively solves both problems of substrate layer porosity and inadequate heat extraction from the bulk layer. Such a substrate system would serve to eliminate or significantly reduce both porosity in the substrate layer and reduce grain size and constituent particle size in the bulk layer by providing more efficient heat extraction rates to the bulk layer region.